1. Field of the Invention
The present invention relates to a projection-type image display apparatus using a high pressure discharge lamp as a light source, a lighting apparatus, and a lighting method.
2. Description of the Related Art
High pressure discharge lamps (hereinafter, simply called “lamps”) used as light sources in projection-type image display apparatuses include a discharge container, a light-emitting material enclosed in a discharge space in the discharge container, and a pair of electrodes provided such that the ends thereof oppose each other in the discharge space. In addition to the light-emitting material, a halide is enclosed in the discharge space, and the halogen cycle that occurs during sustained lighting is used to suppress the consumption of the electrodes. In such lamps, electrode material evaporates off of the electrode tips due to the heat load from the discharge arc, and in the halogen cycle, the evaporated electrode material returns to the electrode tips.
In such lamps that use the halogen cycle, it is necessary to create a proper condition for the halogen cycle that occurs during sustained lighting to function effectively. In order to achieve this, various lighting control patterns including combinations of lighting power, lighting frequency (i.e., current frequency) and lighting current waveform applied to the lamps have been determined, the amount of mercury, halides and the like enclosed in the discharge space has been determined, the distance between the electrodes has been determined, etc. Note that tungsten is often used as the electrode material.
As the cumulative time for which lamp lighting has been sustained (hereinafter, called the “cumulative lighting time”) increases, the amount of electrode material that evaporates becomes greater than the amount of electrode material that returns due to the halogen cycle, as a result of which the electrodes gradually recede (i.e., are consumed), the interelectrode distance increases, and the position of the discharge arc shifts.
An increase in interelectrode distance and a shift in the position of the discharge arc cause a shift away from the position relative to the reflecting mirror that is set based on the original interelectrode distance, and this shift in position relative to the reflecting mirror causes the reflecting effectiveness to attenuate. As a result, the screen illuminance decreases and lamp lifetime (determined to be reached when the illuminance is approximately 50% of the original illuminance) decreases.
Examples of technology for extending the lifetime of such lamps include performing lighting at a lighting power less than or equal to the rated power value during the initial lifetime stage (while the cumulative lighting time is short) (e.g., Japanese Patent Application Publication No. 2000-28988), and changing the frequency and the range in which the power varies (power amplitude) according to the lamp voltage during lamp lighting (e.g., Japanese Patent Application Publication No. 2005-209572).
However, although continually or periodically performing lighting at a low power according to the above technology reduces the heat load on the lamp and realizes and extended lamp lifetime, it is hard to say that a sufficient brightness is ensured during operation of the lamp at the rated power.